In a planar optical waveguide it is common for a substrate material to support an isolation material which in turn supports the waveguide material. Light propagating through the waveguide may be coupled through the isolation material into the lossy substrate. This is a common occurance when the substrate has a high refractive index such as with silicon (Si), gallium-arsenide (GaAs), or lithium niobate (LiNbO.sub.3). Therefore, the thickness of the isolation material must be made sufficient so that the effect of the substrate on waveguide mode properties is low. Typically, for a glass waveguide, a silicon dioxide (SiO.sub.2) thickness of approximately 1 .mu.m has been considered sufficient thickness to substantially provide isolation. Ideally, it is desirable for rays of light to be propagated in the waveguide at angles that provide total internal reflection from both of the waveguide film surfaces. This does not always occur, however, due to such factors as roughness on the isolation material or waveguide interface surfaces, or from inhomogenities within the waveguide, for example. Thus some energy associated with the rays will be transmitted to the substrate or escape into the air during transmission and can significantly reduce the beam strength at the output end of the waveguide. The low loss planar waveguide reduces these power losses, providing a high optical quality path for transmitted light rays.